Dynamic radiography

ABSTRACT

A dynamic radiography system for examining an optically opaque object containing radiation scattering centers by irradiating the object with penetrating radiation to cause radiation scattering by these scattering centers, causing periodic vibration of these scattering centers, detecting radiation scattered by the scattering centers in the object along at least two noncongruent detection channels which are at an angle with each other and which intersect at a common volume within the object, and correlating the outputs of the detectors with a frequency parameter of the vibration to derive thereby information of internal physical and mechanical characteristics of the object.

United States Patent [1 1 Kenney et a1.

[451 Oct. 30, 1973 DYNAMIC RADIOGRAPHY [73] Assignee: ResearchCorporation, New York,

[22] Filed: Aug. 25, 1971 [21] Appl. No.: 174,739

3,422,264 1/1969 Martina 250/61 2,997,586 8/1961 Scherbatskoy...250/71.5 3,361,911 1/1968 Kowalczynski 250/83.3 3,376,415 4/1968Krogstad et a1 250/5l.5

6/1971 Bramlet ..250/7l.5

OTHER PUBLICATIONS Ultrasonic Treatment of Specimens in the ElectronMicroscope by B. Langenecker from The Review of Scientific InstrumentsVol. 37, No. 1, Jan. 1966, pages 103-106,

Primary ExaminerWilliam F. Lindquist Attorney-Robert SQDunham et a1.

[57] ABSTRACT A dynamic radiography system for examining an opticallyopaque object containing radiation scattering centers by irradiating theobject with penetrating radiation to cause radiation scattering by thesescattering centers, causing periodic vibration of these scatteringcenters, detecting radiation scattered by the scattering centers in theobject along at least two noncongruent detection channels which are atan angle with each other and which intersect at a common volume withinthe object, and correlating the outputs of the detectors with afrequency parameter of the vibration to derive thereby information ofinternal physical and mechanical characteristics of the object.

28 Claims, 13 Drawing Figures 3 D SCAN M/DEX/NG S/GNAL (ORRHA 70R V/BRAr/ow Sou/3C5 Z0 D/SPMW DYNAMIC RADIOGRAPHY BACKGROUND OF THE INVENTIONThe invention is in the field of radiography related to examiningobjects which are opaque to visible light, but are transparent tocertain types of radiation. In particular, the invention relates todynamic radiography and involves utilizing radiation scattered from theinteraction between a radiation field and a phonon field coexistingwithin an object, and involves utilizing correlative techniques forenhancing desirable parameters of the scattered radiation.

When a beam of radiation, such as X-ray gamma ray radiation neutrons andthe like, is transmitted through any heterogeneous object, it isdifferentially absorbed, depending upon the varying thickness, densityand chemical composition of the object. The image registered by theemergent radiation on a photographic film adjacent to the object underexamination constitutes a shadowgraph or radiograph of the interior ofthe object. Among the many objects which are commonly examined byradiography are biological specimens, such as the human chest, humanteeth, and major passageways of the human body, and industrial specimenssuch as metal castings, pipes, plates, complex mechanical devices,rubber tires and welds to detect internal physical discontinuities suchas voids, cracks, flaws, segregations, porosities, inclusions and otherdiscontinuities.

The key to conventional radiography is differential absorption ofradiation where variations in thickness, density and chemicalcomposition provide differing attenuation for the penetrating radiation.While such variations are well pronounced in many cases and aresusceptible to detection by conventional radiography, there are manycases where no significant composition, thickness or density differencesexists, as for example in the case of metal castings having hairlinecracks which are normal to the penetrating radiation beam, or in thecase of many internal organs of the human body. Thus, where nosignificant thickness density or composition differences exist in anobject, conventional radiography can not be used successfully, and theneed exists for better methods of examining such objects.

One specific example of the use of conventional penetrating radiationradiography to detect discontinuities in optically opaque objects isdisclosed in Bernstein U.S. Pat. No. 3,158,744 where two detectorspositioned close to each other look at a beam of hard X-ray radiationwhich has traversed an optically opaque object. When the beam encountersa discontinuity in the object, differing amounts of penetratingradiation may reach the two detectors. The disclosure in Bernstein islimited to the use of straight line penetrating radiation and no mentionis made of use of scattered radiation. In conventional radiography,scattered radiation, which is radiation traveling in directions otherthan straight through the object which is being examined, is consideredundesirable because it results in fogging and poor definition of theradiograph. Generally, the effects of scattered radiation are minimizedby the use of lead screens or diaphragms consisting of a grid of closelyspaced parallel lead sheets which cut off side scattering when moved ina position parallel to the plane of the film during exposure.

Scattered radiation has been used, however, for the purposes ofdetermining density and mass of objects. For example, Scherbatskoy, U.S.Pat. No. 2,997,586,

discloses the use of measured variations of scattered radiation todetermine variations in the density of an object. Kowalczynski, U.S.Pat. No. 3,361,911, shows measuring the mass of an object by detectingscattered radiation; and Martina, U.S. Pat. No. 3,422,264, disclosesgeneration of stereoscopic radiographs by means of irradiating an objectwith a neutron beam to induce a secondary emission of gamma-ray photonswhich are detected by two pinhole cameras looking at the object at anangle to each other. The last mentioned three pa tents do not discloseutilizing a radiation field-phonon field interaction for the purpose ofdetecting internal physical characteristics of optically opaque objects,and do not solve the problem of deriving satisfactory one, two or threedimensional information of the internal physical characteristics ofoptically opaque objects in which no significant thickness, density orcomposition differences exist. There is still a need for obtaining suchinformation, both for industrial uses and for physiological uses.

SUMMARY OF THE INVENTION The invention is in the field of radiographicexamination of objects which are optically opaque. In particular, theinvention relates to dynamic radiography which involves detecting andcorrelating radiation scattered from an object in which an interactionof a radiation field and a phonon field takes place, said detecting andcorrelating being carried out for the purpose of obtaining one, two, orthree-dimensional information about internal physical and mechanicalcharacteristics of the object.

In its broadest aspect, the invention relates to causing a radiationfieldphonon field interaction in an object and detecting radiationscattered by scattering centers in the object to generate informationindicative of internal physical and mechanical characteristics of theobject. The radiation field may be caused, for example, by irradiatingthe object with radiation such as gamma rays, X-rays, electrons,neutrons, alpha-particles, or the like; and the phonon field may becaused, for example, by vibrating the object at a sonic frequency bycausing other types of phonon perturbations or by relying on naturallyexisting perturbations. When two detectors look at the object along twodetection channels intersecting at a common volume in the object, andwhen the object is continuous at that common volume, the motion ofscattering centers in that volume is uniform; therefore the detectionsof scattered radiation along the two detection channels result in twostatistically independent variables which have low cross-correlation.If, however, a discontinuity exists within the common volume at whichthe two detectors are looking, a discrete step in the amount ofscattering occurs within that volume, and the scattering detected alongthe two detection channels undergoes a common change keyed to the phononfield perturbations. When the outputs of A single detector of scatteredradiation may be used, instead of the two detectors described above, ifonly one-dimensional or two-dimensional information about the internalphysical and mechanical characteristics of the object is desired.

The object under dynamic radiography examination may be scanned byestablishing suitable relative motion between the object and thedetectors, or by imaging the objects by means of scattered radiation ondetection screens. These detection screens may feed video scanningsystem cameras which are synchronized in their scan patterns to acceptdata in sequence from the entire object of from a portion of the object.Selected pairs of points of the two images on the two detection screensidentify two intersecting narrow beams of scattered radiation whichoriginate from a common volume in the object.

An important property of a dynamic radiograph generated as disclosed inthis specification is the fact that it can reflect the mechanicalresponse of the object to with respect to the radiation, as for exampleby vibrating the object or by otherwise causing a phonon field in theobject; detecting means for obtaining a measure of the radiationscattered by the object in each of two different detection channels, asfor example, by using a pair of scintillation detectors or by using apair of detection screens; and means for correlating the measures ofradiation in the two channels, as for example, by using a conventionalcorrelator connected to the outputs of the two channels. The standardcorrelator may include as a third input the frequency for the means ofvibrating the object; and, the object may be scanned to obtainthree-dimensional information of its internal characteristics. A singledetector may be used if threedimensional information is not essential.

The invention provides highly improved resolution in cases whereconventional radiography may also be used, and provides one, two orthree-dimensional information of objects which may not be examined byconventional radiography. For example, the invented dynamic radiographymay be used for physiological examinations of organs which do notprovide sufficient radiation attenuation contrast and therefor cannot beconveniently examined by conventional radiography, and may be used todetect defects in industrial objects such as castings, etc. whichdefects are so small with respect to the object size that they cannotprovide attenuation contrast detectable by conventional radiography.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration ofa dynamic radiography system for examining an optically opaque object bydetecting radiation scattered along two detection channels at an angleto each other.

FIGS. 2a, 2b, and 2c are diagrammatic illustrations of the detectionchannels in different relative positions to a discontinuity within theobject under radiographic examination.

FIGS. 3a, 3b, and 3c illustrate the outputs of components of the systemof FIG. 1 under the conditions illustrated in FIGS. 20, 2b, and 20respectively.

FIG. 4 illustrates the use of the dynamic radiography system of FIG.Ifor examining an object having a naturally occurring phonon field.

FIG. 5 illustrates a system for dynamic radiography of an object whichincludes a radioactive tracer.

FIG. 6 illustrates the use of a dynamic radiography system for providingtime history of a diffusing process.

FIG. '7 illustrates a dynamic radiography system utilizing detectionscreens and electronic scanning of the object by means of scanning theimage on the detection screens.

FIG. 8 is an illustration of the principles involves in the scanningcarried out by the system of FIG. 7.

FIG. 9 is a schematic blown up illustration ofa detection system forminga part of the embodiment of FIG. 7.

DETAILED DESCRIPTION The basic principles of using a radiationfield-phonon field interaction and of correlating scattered radiationparameters and phonon field parameters to obtain three-dimensionalinformation about the internal physical or mechanical characteristics ofoptically opaque objects are illustrated in the exemplary embodiment ofFIG. 1 which shows the examination of an object 10 containing aninternal flaw 12. A radiation field is established in a portion of theobject It) by means of a radiation beam 14 generated by a suitablycollimated radiation source 15. A phonon field is established in theobject v10 by means of a vibrator I6 driven by a motor 18 energized tocause sonic frequency vibration under the control of a vibration source20. Radiation scattered by the object 10 is detected by means of twodetectors 22 and 24 which are suitably collimated to receive scatteredradiation along detection channels 26 and 28 respectively whichintersect at a common volume 30 which is inside the object 10 and is onthe beam 14 from the radiation source 15. The signals from the detectors22 and 24 and the signal from the vibration source 20 are correlated bya signal correlator 32, and the output of the correlator 32 is displayedat a display 34.

The radiation source 15 may be a conventional X-ray source having asuitable level output, but may alternately be a source of otherradiation that may be compatible with the object 10, such as neutron,gamma ray, electron, alpha-particle or other radiation capable ofcausing scattered radiation in at least a portion of the object 10. Theradiation source 15a is suitably collimated by a collimator 35b to emita narrow beam I4. The vibrator 16 may be a conventional shaking tablecapable of vibrating the object It) at sonic frequency, for example, atfrequencies in the range of between 7% Hertz and 1,000 I-Iertz at up toone millimeter excursion when suitably driven by the motor 18. Thevibration source 20 may be a suitably amplified audio oscillator. Thedetectors 22 and 24 may be conventional photomultiplier scintillationcrystal units combined with integration networks to provide a currentsuitably proportional to the average detection rate of scatteringevents. The signal correlator 32 may be a standard analogue device forcarrying out correlation, such as crosscorrelation, of the signals fromthe detectors 22 and 24 and the audio frequency signal from thevibration source 20. The display 34 may be a conventional XY plotterwhich plots the output of the signal correlator 32 on an XY coordinatesystem representing the volume 3% with respect to a plane in the object10.

In operation, the radiation source generates a narrow beam I4 ofpenetrating radiation which impinges on and penetrates the object It) atleast through the finite common volume at which the detector channels 26and 28 intersect. Interaction between incident radiation from theradiation source 15 and scattering centers in the volume 30 results inthe generation of scattered radiation emitted from the volume 30. Someof this scattered radiation is emitted along the detection channel 26and is detected by the detector 22, and some is emitted along thedetection channel 28 and is detected by the detector 24. Since the twodetection channels are at an angle to each other, each detector detectsscattering events which are independent of the scattering eventsdetected by the other detector. Thus, if the finite volume 30 enclosesonly a homogenous collection of scattering centers, each detector willdetect a series of scattering events independent of the series ofscattering events detected by the other detector. The output of eachdetector includes, in addition to the signal resulting from scatteringevents, a noise component which is also statistically independent of thenoise component in the signal from the other detector.

Thus, if the two detectors 22 and 24 are looking at a common volume 30which includes only a continuous and homogenous distribution ofscattering centers, the outputs of the two detectors are statisticallyindependent and a cross-correlation of the outputs of the two detectors,which cross-correlation serves as a measure of the dependence betweentwo statistical variables, has a low value.

If the collection of scattering centers in the finite volume 30undergoes some phonon perturbation, such as if the collection ofscattering centers within the volume 30 is vibrated with respect to thebeam 14 from the radiation source 15, and if there is no discontinuityof scattering centers in the volume 30, the phonon perturbation causescontinuous motion of scattering centers, and the scattering probabilitydoes not change due to the phonon field.

If, however, there is a discontinuity within the finite volume 30, suchas if the finite volume 30 includes a wall separating solid metal froman air bubble, and if the metal is vibrated such that the wall betweenthe air bubble and the solid metal jumps in and out of the finite volume30, the frequency of vibration shows in the outputs of the detectors 22and 24 because of the difference in scattering probability between thetime when the common volume 30 includes only solid metal and the timewhen the common volume 30 includes part solid metal and part air bubble.Under such conditions, the outputs of the detectors 22 and 24 are nolonger statistically independent, but have a common charac teristic,namely, a change in detected scattering events keyed to the frequency ofvibrating the material within the common volume 30. A cross-correlationof the outputs of the detectors 22 and 24 and of a signal representingthe phonon field frequency has a high value because it is a measure ofthe interdependence of the correlated signals.

For a qualitative explanation of detecting a discontinuity, such as anair bubble, in the object 10, reference is made to FIGS. 2a, 2b and 20which show a partial sectional view through FIG. 1 taken along the axisof the detection channels 26, 28 and to FIGS. 3a, 3h, and 30 which showthe outputs of the detectors 22 and 24, the phonon field frequency andthe output of the correlator 32 under the conditions of FIGS. 2a, 2b and20 respectively.

Referring to FIG. 2a, assume that the object 10 is a metal block, thatthe common volume 30 at which the two detectors 22 and 24 look includesonly solid, homogenous and continuous metal and that the flaw 12 is anair bubble which is just outside the common volume 30. Further assumethat the air bubble 12 moves due to the phonon field excitation of theobject I0 between two extreme positions the position shown as a solidline circle and the position shown as a broken line circle, and that ineach position, the air bubble is completely outside the common volume36). Assume that the solid line circle corresponds to the positive peakof the phonon field frequency curve shown in FIG. 3a and the broken linecircle representation of the air bubble 12 corresponds to the negativepeak of the phonon field frequency curve. Since the common volume 30contains no discontinuities, the outputs of the detectors 22 and 24 eachinclude a component resulting from detected scattering events and acomponent resulting from noise. The two components of the signal outputfrom the detectors 22 are statistically independent of the twocomponents output from the detector 24. Therefor, the output of thecorrelator 32 is essentially a flat zero signal. The fact that thescattering centers within the common volume 30 move with respect to thedetection channels 26 and 28 has no effect on the correlation betweenthe signal output of the detectors 22 and 24.

Referring to FIG. 2b, assume that the phonon field exciting the object10 moves the air bubble 12. from a position just outside the commonvolume 30 to a position partly inside the common volume 30, and that theposition outside the common volume 30 (shown as a solid line circle)corresponds to the positive peak of the phonon field frequency curve andthe position partly inside the common volume 30 (shown as a brokencircle) corresponds to the negative peak of the phonon field frequencycurve shown in FIG. 3b. When the air bubble 12 is outside the commonvolume 30, the situation is the same as that described in connectionwith FIGS. 20 and 3a, namely, the outputs of the detectors 22 and 24 arestatistically independent, and the output of the correlator 32 isessentially zero. When the air bubble l2 enters the common volume 30 dueto the phonon field excitation of the object 10, the scatteringprobability starts undergoing a change due to the change in thecomposition of the common volume 30. If the air bubble 12 generates lessscattering than the surrounding metal, then the output of the detectors22 and 24 starts dropping and continues dropping as more of the airbubble I2 goes into the common volume 30 due to the phonon fieldexcitation of the object 10. When the air bubble 12 is furthest into thecommon volume 30, at the negative peak of the phonon field frequencycurve, the outputs of the detectors 22 and 24 are concurrently at theirlowest value, and the output of the correlator 32, which carries out acrosscorrelation function, is at its peak because it is a measure of thelikeness between the outputs of the detectors 22 and 24 and the phononfield frequency curve. When the air bubble 12 starts returning towardits position outside the common volume 30, the order of events isreversed, and when the bubble 12 is completely outside the common volume30, the situation reverts to that described in connection with FIGS. 2aand 3a when the outputs of the detectors 22 and 24 are statisticallyindependent and the correlation therebetween is essentially zero. As thenext negative portion of the phonon field frequency curve starts, thecycle of events is repeated and the output. of the correlator 32 againstarts going high.

As a third situation, assume that the air bubble 12 is completely insidethe common volume 30 at which the two detectors 22 and 24 look, and thatthe phonon field which excites the object moves the air bubble 12 onlythrough positions completely inside the volume 30. Now the phonon fieldcauses no change in the consistency of the common volume 30 and theoutputs of the two detectors 22 and 24 are again statisticallyindependent, just as in the case of the situation described inconnection with FlGS. 2a and 3a.

It is noted that correlation operations other than cross-correlation maybe useful. For example, simple signal averaging, or a measuring of ACsignal voltage may be useful in utilizing dynamic radiography. In oneexample where X-ray radiation has been scattered from two-phasewater-air flow in a vertical pipe, the AC signal voltage has been foundto be a good indication of the quantity of air injected into the waterflow.

The object If) may be scanned by the common volume 30 at which thedetectors 22 and 24 look simultaneously by establishing a relativescanning motion between, on the one hand the object 10, and on the otherhand, the combination of the radiation source 15 and the detectors 22and 24. For example, a three dimensional scan indexing drive 36 may beused to drive the combination of the radiation source 15 and thedetectors 22 and 24 such that the object 10 is examined slice by sliceand the output of the correlator 32 is displayed on the display 34 as aseries of successive XY plots of the internal physical and mechanicalcharacteristics of successive slices. For example, a rectilinear scanmay be started by positioning the common volume 30 at which thedetection channels 26 and 28 and the radiation beam I4 intersect at onecorner of the object 10 and then moving that common volume 30rectilinearly to scan along coplanar successive lines of a slice of theobject l0 parallel to a side thereof, and then moving on to a next sliceparallel to the first one until the entire volume of the object 10 isscanned in that fashion. When, in the course of scanning, the commonvolume 30 approaches a flaw 12, the situation resembles that explainedin connection with FIGS. 2a and 3a. When the common volume 30 movescloser to the flaw l2 and the flaw I2 starts moving in and out of thecommon volume 30 as the object 10 is vibrated, the situation resemblesthat explained in connection with FIGS. 21; and 3b. If the flaw I2 issmall enough to be completely included within the volume 30, as shown inFIG. 30, the situation resembles that described in connection with FIGS.2c and 3c. As the flaw 12 starts movingout of the common volume 30,again signals similar to those shown in FIG. 3b appear at the outputs ofthe detectors 22 and 24 and of the correlator 32. i

If the display 34 (FIG. 1) is an XY plot of the scanning pattern of aslice of the object 10, with a scan line of the display 34 correspondingto a scan line through the slice of the object 10, and if the output ofthe correlator 32 is used to intensity-modulate the scan lines on thedisplay 34, then the display 34 may be a light surface except for darkspots corresponding to a high output of the correlator 32 which in turncorrespond to discontinuities in the object 10. The position of thediscontinuity within the object I0 is uniquely determined threedimensionally by the intersection of the detection channels 26 and 28.Note that the display 34 is synchronized with the scan indexing drive 36to establish a unique one-to-one correspondence between a position ofthe common volume 30 within the object 10 and a point displayed on aparticular X-Y plot of the display 34.

It is noted that scan speed is related to the necessary detection timefor producing usable signals in response to detected scatteredradiation. Asone example, for the mechanical scan system describedabove, scanning speeds of the order of one millimeter per minute to 10centimeters per second may be useful.

There is a defined relationship between the radiation field density, thephonon field frequency, the scan speed and resolution. For example, fora given radiation field density, the upper limit of phonon fieldfrequency is reached when a cycle can not be resolved with the availablescattered and detected events. The number of scattered and detectedevents needed to resolve a cycle is of the order of one hundred. Fordiagnostic X-ray apparatus utilizing dynamic radiography, phonon fieldfrequencies of up to 1,000 Hz may be usable. Resolution dependsprimarily on the collimator aperture. The amplitude of the displacementof scattering centers (clue to the phonon field) with respect to thedetection channels is an important factor. If the amplitude is toolarge, the resulting image is diffused; if the amplitude is too small,it may be difficult to observe the fluctuations in scattered radiationfield amplitude.

If the object which is to be examined by dynamic radiography isnaturally under the effect of a phonon field, it is unnecessary toinduce therein additional phonon fields as described in connection withFIG. I. One example of a naturally existing phonon field is illustratedin FIG. 4 which shows in cross section a pipe carrying a two-phaseturbulent flow containing a liquid phase 40 and a gas phase 42.Components of the apparatus shown in FIG. 4 which are like components ofthe apparatus of FIG. 1 are designated by like reference numerals. Whenthe common volume 30 at which the detectors 2 2 and 24 look is entirelywithin the liquid phase 40, at all times phonon field perturbations(causing motion of the liquid-gas interface 41) have no effect on thestatistical independence between the outputs of the detectors 22 and 24,and the output of the signal correlator 32 which carries outcross-correlation is essentially zero. Similarly, when the common volume30 is entirely within the gas phase 42, at all times phonon fieldperturbations of the gas phase have no effect on the statisticalindependence between the outputs of the detectors 22 and 24, and theoutput of the signal correlator 32 is again essentially zero. When,however, the common volume 30 includes the interface 41 be tween theliquid phase 40 and the gas phase 42, motion of the interface 41 withrespect to the common volume 30 due to the phonon perturbationsresulting from the turbulent flow cause a common change in thescattering events detected by the detectors 22 and 24, and this commonchange is detected by the signal correlator 32 which generates a signaldifferent from zero, because of detector output cross-correlationsimilar to that shown in FIG. 31;, but without the periodicity due tovibrational phonon field. The system comprising the liquid phase 40 andthe gas phase 42 may be scanned by the common volume 30 by utilizing ascan indexing drive 36 to cause a two dimensional scan, in the plane ofFIG. 4 of the cross-section of the pipe 38. The display 34 issynchronized with the scan indexing drive 36 to represent thecross-section of the pipe 38 shown in FIG. 4 by showing a blank area foreach of the liquid phase 46 and the 'gas phase 42 and showing a darkline for the interface 41.

In case the object 10 which is examined by dynamic radiography containsa radioactive tracer, an external source of radiation, such as thesource (FIG. I and 4) may be unnecessary. One example of a system inwhich a radioactive tracer is relied on for generating emission ofradiation and scattering events is illustrated in FIG. 5 which shows asystem identical to that shown in FIG. 4 except that there is noexternal radiation source 15, and either the gas phase 42, or the liquidphase 44), or both, contain a radioactive tracer which emits radiationresulting in scattering events. As in the apparatus of FIG. 4, when thecommon volume 30 in FIG. 5 contains only homogenous gas phase at alltimes, the scattered and emitted events detected by the detectors 22 and24 are statistically independent; therefore the outputs of the detectors22 and 24 are two statistically independent variables and the output ofthe signal correlator 32 is essentially zero. Similarly, when the commonvolume 30 includes only homogenous liquid of the liquid phase 40, theoutputs of the detectors 22 and 24 are two statistically independentvariables and the output of the signal correlator 32 is againessentially zero. When, however, the common volume 30 includes theinterface 4]. between the liquid phase 443 and the gas phase 42, theperturbations due to the turbulent flow cause variation in thescattering and emission characteristics of scattering centers within thevolume 30 with respect to the volume 30. These phonon fieldperturbations affect simultaneously and similarly each of the detectors22 and 24. The detected perturbations of the phonon field introduce acommon characteristic into the outputs of the detectors 22 and 24,namely, the effects of the phonon field perturbation on scatteringcenters and hence on scattering events detected by each of the detectors22 and 24.

Thus, when the common volume 30 at which the detectors 22 and 24 looksimultaneously includes the interface 41 between the liquid phase 40 andgas phase 42, the output of the signal correlator 32 is high, while whenthe common volume 30 includes either only the gas phase 42 or only theliquid phase 40, the output of the correlator 42 is low. The showncross-section of the pipe 38 may be scanned by means of a scan indexingdrive 36 which is synchronized with the display 34 such that the display34 shows a plot of the cross section scanned by the common volume 30.Any convenient scanning pattern may be used, such as a rectilinear scansimilar to the manner in which a television tube screen is scanned, or apolar-coordinate scan. The display 34 is scanned in the same manner inwhich the shown cross section of the pipe 38 is scanned. The scan lineson the display 34 may be intensity-modulated by the output of the signalcorrelator 32 such that low level signals from the correlator 32 resultin lightly traced lines on the display 34 and high level signals fromthe correlator 32 result in heavily traced lines on the display 34.Under such condition, the image displayed on the display 34 is areproduction of the interface 41 between the liquid phase 40 and the gasphase 42. If the interface between the liquid phase 40 and the pipeinner face 38a is included in the scan pattern of the scan indexingdrive 36, then the display 34 would also show a circle corresponding tothat inner face 38a.

In addition to detecting discontinuities of the type described inconnection with FIGS. ll, 4 and 5, dynamic radiography may be used toprovide information of the time history of processes such as, forexample, diffusion processes. An illustration of such use is shown inFIG. 6 where a conduit such as a pipe 38 contains a fluid 40 flowing inthe indicated direction and a diffusing material source 44 whichdiffuses into the fluid 40 a diffusion material containing aradiographic tracer. If the common volume 30 at which the detectionchannels 26 and 28 of the detectors 22 and 24 respectively intersect isheld stationary with respect to the conduit 38, then the motion of thediffusion process provides, at the outputs of the detectors 22 and 24, acommon signal component which is correlated by the signal correlator 32and, when displayed on the display 34 as a function of time, provides atime history of the diffusion process. Alternately, a periodic phononfield may be generated by a sonic frequency oscillator 46 vibratingwithin the fluid 40 and mechanically coupled therewith to causedisplacement of scattering centers within the fluid 40 with respect tothe common volume 30. A scan of the fluid 40 may be effected by causingrelative motion between the common volume 30 at which the two detectors22 and 24 look and the conduit 38 by means of a scan indexing drive 36scanning the conduit 38 in a suitable patternand synchronized with thedisplay 34 for the purpose of establishing a oneto-one correspondencebetween points inside the conduit 38 and points on the display 34.

In order to scan rapidly an object under dynamic radiography, themechanical scanning of the object described in connection with FIGS. ll,4, 5 and 6 may be replaced by electronic scanning as illustrated in theexemplary embodiment of FIG. '7. in electronic scanning, a source ofradiation is kept flxed with respect to the object under examination andthe object is imaged by scattered radiation on a detection systemcomprising detection screens coupled to image intensifiers. Thedetection system is viewed by video cameras to accomplish scanning. Theobject is swept in threedimensional format. The scattered radiation bywhich the object is imaged on the detection system is collimated for thepurpose of identifying the origin of scattered radiation from each pointof the object. The scans are synchronized to interrogate each pointinside the object in its turn.

Referring to FIG. 7, the object 10 is irradiated by means of a radiationsource 50 which generates not a narrow beam of radiation as theradiation source 15 described earlier, but a wide beam 53 whichirradiates either the entire object it or a substantial portion thereof.If the object 10 is not naturally under the effects of a phonon field, aphonon field may be introduced by means of a vibrator or by othersuitable means. The radiation beam 51 from the radiation source 50interacts with scattering centers in the object l0, and the scatteredradiation is detected simultaneously by a detection system 52 collimatedby means of a dynamic collimator 56 known in the art as a Bucky Plate,and by a detection system 54'collimated by a Bucky Plate 58. Each of theBucky Plates 56 and 58 comprises a grid of perforations and allowsscattered radiation to reach the image intensifiers only through theperforations. A pair of perforations, one on each Bucky plate, definestwo detection channels intersecting at a common small volume within theobject 10. For example, the pair of perforations 56a in the Bucky plate56 and 58a in the Bucky plate 58 define respectively a detection channel26 and a detection channel 28 which intersect in a common volume 30inthe-object i0. Scattered radiation along the detection channel 26 passesthrough the perforation 56a in the Bucky plate 56 and is imaged on adefined point on the detection system 52. Similarly, scattered radiationalong the detection channel 28 passes through the perforation 580 on theBucky plate 58 and is imaged on a defined point on the detection system54. These two defined points on the image intensifiers 52 and 54constitute a selected pair of points, each measuring radiation scatteredby the common volume 30 within the object 10. Other selected pairs ofpoints, one point from each of the image intensifiers S2 and 54,constitute a measure of the radiation emitted from different definedcommon volumes 30 within the object 10. If the entire object i is imagedon each of the detection system 52 and 54, then any small common volume30 within the object if) is defined by a selected pair of points, onefrom each of the image intensifiers 52 and 54, and a one, two orthree-dimensional scan of the object l0 may be carried out bysimultaneously scanning each of the detection systems 52 and 54 twodimensionally in suitable synchronism by means of conventional videocameras 53 and 55 respectively.

For an illustration of scanning the object by electronically scanningthe image intensifiers 52 and 54, reference is made to FIG. 8 whichshows diagramatically a partial sectional view of an image on thedetection systems 52 and 54 and of the object 10 viewed from the top ofthe object it in FIG. 7. Assume that the entire object it? is irradiatedand that the entire object M] is imaged by scattered radiation on eachof the detection systems 52 and 54. Then, the radiation scattered by acommon volume 30a is defined by a selected pair of points 520 and 54a onthe detection systems 52 and 5d respectively; the radiation scattered bya common volume 36b is defined by points 52b and 54b on the detectionsystems 52 and 54 respectively; and the radiation scattered by stillanother common volume 30c is defined by points 52c and 540 on thedetection systems 52 and 54 respectively. When the detection systems 52and 54 are scanned electronically such that troducing suitable delaysinthe scan patterns of scanning the detection systems 52 and 54, by meansof conventional video camera techniques, the common volume defined by aselected pair of points on the detection system image can be movedanywhere within the plane defined by the points shown on the detectionsystems 52 and 54. For example, the common volume designated MM isexamined by scanning such that points 520 and 54b are read outsimultaneously.

The detection systems 52 and 54 are scanned by the video cameras 53 and55 respectively under the control of a suitably programmed centralprocessor for properly correlating selected pairs of points of thedetection system images and for extracting desired informationtherefrom. The'output of the central processor 60 may be utilized by adata printout to show the location of discontinuities within the objectll) and by a holographic viewing system 62 which may represent athree-dimensional image of discontinuities within the Object 10.

Referring to FIG. 9, the detection system 52 includes a detection screen52d which detects scattered radia tion collimated by the Bucky Plate 56and image intensifiers 522. The detection screen 52d and the imageintensifiers 52c are scanned electronically by a conventional videocamera 53 in a conventional rectangular raster pattern, or in adifferent pattern that may be chosen by the central processor 60. Thedetection system S4 is similarly constructed and positioned. The scanpatterns and the pattern timing of the video cam eras 53 and 55 aresynchronized such that any plane within the object 10 may be scanned bychoosing suitable time delays between the scan beams of the videocameras. Alternately, the scan information may be digitized and storedin the memory of the central processor 6%, and a view of a selectedpoint, plane, or volume of the object 10 may be generated by suitablyextracting portions of the stored digitized information.

It is noted that the use of a single detector may be satisfactory insome applications of dynamic radiography. For example, a single detector22 may be used in the embodiments of FIGS. 4 and 5 to examine two-phaseflow, or in the embodiment of FIG. 6 to examine a diffusion process. Ofcourse, three-dimensional information is not generated, rather anaverage weighted by radiation transport probabilities is detected by thesingle detector. Examples of uses of a dynamic radiography systemshaving only a single detector include: steam quality monitoring,airborne particulate monitoring, product size monitoring in crushing orother bulk forming operations, and liquid-solid, liquid-gaseous, orsolid-gaseous mixture measurements or interface motion.

An important aspect of dynamic radiography is the contrast developed byvarying amplitude of mechanical vibration response of the object. As oneexample, one medical use of dynamic radiography is the detection of lungand lung passageway disease. Certain pulmonary disorders result inchanged mechanical properties of lung tissue, and certain such changesmay provide contrasts in the' intensity of dynamic radiographs of lungwalls. Similar situations exist for certain coronaryarterial disorders.Dynamic radiography provides detection capabilities not only forparameters such as physical discontinuities within an optically opaqueobject, but also for changes in the mechanical response of an object tophonon fields.

'What is claimed is:

1. Apparatus for examining an optically opaque object containingpenetrating radiation scattering centers, comprising: 7

a. means for irradiating the object with penetrating radiation to causeradiation scattering by scattering centers in the object;

b. means for causing periodic vibration of said scattering centerswithin the object;

c. means for detecting radiation scattered by scattering centers in theobject along at least two noncongruent detection channels which are atan angle with each other and which intersect at a common volume withinthe object; and

d. means for correlating information generated by said detecting meansin response to detected scattered radiation with informationrepresenting the periodic vibration of said scattering centers withinthe object to derive thereby information of internal physical andmechanical characteristics of the object.

2. Apparatus as in claim 1 including means for causing a scan of theobject by said common volume.

3. Apparatus as in claim 2 wherein said scan causing means comprisemeans for causing relative motion between .the detection means and theobject.

4. Apparatus as in claim 3 wherein said relative motion between thedetecting means and the object is along a defined scanning pattern.

5. Apparatus as in claim 1 wherein the detecting means comprise twodetection systems each providing an image of the radiation scatteed fromat least a portion of the object along one of the two directions definedby said detection channels, with selected pairs of points, one on eachdetection system image, defining radiation scattered from a commonvolume of the object 6. Apparatus as in claim 5 including means forscanning the object comprising means for successively examiningdifferent selected pairs of points on the detection system images tosimulate a scan of the object.

7. Apparatus as in claim 6 wherein the correlating means include meansfor correlating parameters of the selected pairs of points with thevibrating means to enhance differences between different pairs ofpoints.

8. Apparatus as in claim 7 wherein the scanning means include videocamera means for electronically scanning the detection system images ina synchronism simulating a scan of the object.

9. Apparatus as in claim 1 wherein the irradiating means comprise aradiation source located outside the object and generating a radiationbeam impinging on, and at least partly penetrating the object.

10. Apparatus as in claim 1 wherein the irradiating means comprise aradiation source located within the object and generating a radiationbeam.

111. Apparatus as in claim 1 wherein the irradiating means compriseradioactive material interspersed with said scattering centers in theobject.

12. Apparatus as in claim 1 wherein the detection means comprise twodetectors each responsive to scattered radiation along a different oneof said detection channels and generating an output signalrepresentative of detected scattered radiation, and wherein thecorrelating means comprise means for receiving as input signals theoutputs of the two detectors and a signal representative of a frequencyparameter of the ,vibrating means and for generating a correlated outputsignal enhancing the similarities between the three input signals.

13. Apparatus as in claim 12 wherein the correlating means is across-correlator.

14. Apparatus as in claim ll wherein the detecting means comprise twodetection systems each having a screen imaging scattered radiation alonga different one of said two detection channels and each comprising adynamic collimator having a grid of openings each passing a scatteredradiation beam along a portion of the associated detection channel, withpairs of selected openings, one from each detection system, defining acommon volume within the object, and with each opening defining an imagepoint on the detection system screen.

15. Apparatus as in claim 14 including means for scanning the detectionsystem image screens to simulate a scan of the object.

16. Apparatus as in claim l5 wherein the scanning means include meansfor simulating a three dimensional scan of the object.

17. Apparatus as in claim 15 wherein each of the detection systemsincludes an image intensifier forming said image screen and wherein eachdetection system includes a video camera for electronically scanningsaid image screens to simulate a scan of the object.

18. Apparatus as in claim 17 wherein said scanning means include meansfor simulating a threedimensional scan of the object.

19. Method of examining an optically opaque object containingpenetrating radiation scattering centers, comprising the steps of:

a. irradiating the object with penetrating radiation to cause radiationscattering by scattering centers in the object;

b. causing periodic vibration of said scattering centers within theobject;

0. detecting radiation scattered by scattering centers in the objectalong at least two noncongruent detection channels which are at an anglewith each other and which intersect at a common volume within theobject; and

d. correlating information generated by said detecting means in responseto detected scattered radiation with information representing theperiodic vibration of the scattering centers within the object to derivethereby information of internal physical and mechanical characteristicsof the object.

20. Method as in claim 19 including the step of scanning the object bythe common volume at which the two detection channels intersect.

21. Method as in claim 20 wherein said scanning is carried out bycausing relative motion between the detection means and the object.

22. Method as in claim 19 wherein the detecting step includes imagingradiation along each of said detection channels to provide thereby apair of screen images, each screen image representing radiationscattered along one of said detection channels and each having aplurality of points representing radiation scattered along portions ofthe detection channel, with selected pairs'of points, one on eachscreen, defining radiation scattered from a common volume of the object.

23. Method as in claim 22 including the step of successively examiningdifferent selected pairs of points of the screens to simulate a scan ofthe object.

24. Method as in claim 23 wherein the correlating step includescorrelating parameters of the selected pairs of points with thevibrating to enhance differences between different pairs of points.

25. Method of examining an optically opaque object which containspenetrating radiation scattering centers undergoing periodic vibrationalmotion, comprising the steps of:

a. irradiating the object with penetrating radiation to cause radiationscattering by said periodically vibrating scattering centers in theobject;

b. detecting radiation scattered by scattering centers in the objectalong at least two noncongruent detection channels which are at an anglewith each other and which intersect at a common volume within theobject; and

c. correlating information generated by said detecting means in responseto detected scattered radiation with information representing theperiodic vibration of the scattering centers within the object to derivethereby information of internal physical and mechanical characteristicsof the object.

26. Method as in claim 25, including the step of caus ing a scan of theobject by said common volume.

27. Method as in claim 25 wherein the detecting step includes imaging onimage screens radiation scattered along each of said detection channels,with selected pairs of points, one on each detection image screendefining radiation scattered from a common volume of the object.

28. Method as in claim 21 including the step of scanning the object bysuccessively examining different selected pairs of points on the imagescreen to simulate a scan of the object.

1. Apparatus for examining an optically opaque object containingpenetrating radiation scattering centers, comprising: a. means forirradiating the object with penetrating radiation to cause radiationscattering by scattering centers in the object; b. means for causingperiodic vibration of said scattering centers within the object; c.means for detecting radiation scattered by scattering centers in theobject along at least two noncongruent detection channels which are atan angle with each other and which intersect at a common volume withinthe object; and d. means for correlating information generated by saiddetecting means in response to detected scattered radiation withinformation representing the periodic vibration of said scatteringcenters within the object to derive thereby information of internalphysical and mechanical characteristics of the object.
 2. Apparatus asin claim 1 including means for causing a scan of the object by saidcommon volume.
 3. Apparatus as in claim 2 wherein said scan causingmeans comprise means for causing relative motion between the detectionmeans and the object.
 4. Apparatus as in claim 3 wherein said relativemotion between the detecting means and the object is along a definedscanning pattern.
 5. Apparatus as in claim 1 wherein the detecting meanscomprise two detection systems each providing an image of the radiationscatteed from at least a portion of the object along one of the twodirections defined by said detection channels, with selected pairs ofpoints, one on each detection system image, defining radiation scatteredfrom a common volume of the object.
 6. Apparatus as in claim 5 includingmeans for scanning the object comprising means for successivelyexamining different selected pairs of points on the detection systemimages to simulate a scan of the object.
 7. Apparatus as in claim 6wherein the correlating means include means for correlating parametersof the selected pairs of points with the vibrating means to enhancedifferences between different pairs of points.
 8. Apparatus as in claim7 wherein the scanning means include video camera means forelectronically scanning the detection system images in a synchronismsimulating a scan of the object.
 9. Apparatus as in claim 1 wherein theirradiating means comprise a radiation source located outside the objectand generating a radiation beam impinging on, and at least partlypenetrating the object.
 10. Apparatus as in claim 1 wherein theirradiating means comprise a radiation source located within the objectand generating a radiation beam.
 11. Apparatus as in claim 1 wherein theirradiating means comprise radioactive material interspersed with saidscattering centers in the object.
 12. Apparatus as in claim 1 whereinthe detection means comprise two detectors each responsive to scatteredradiation along a different one of said detection channels andgenerating an output signal representative of detected scatteredradiation, and wherein the correlating means comprise means forreceiving as input signals the outputs of the two detectors and a signalrepresentative of a frequency parameter of the vibrating means and forgenerating a correlated output signal enhancing the similarities betweenthe three input signals.
 13. Apparatus as in claim 12 wherein thecorrelating means is a cross-correlator.
 14. Apparatus as in claim 1wherein the detecting means comprise two detection systems each having ascreen imaging scattered radiation along a different one of said twodetection channels and each comprising a dynamic collimator having agrid of openings each passing a scattered radiation beam along a portionof the associated detection channel, with pairs of selected openings,one from each detection system, defining a common volume within theobject, and with each opening defining an image point on the detectionsystem screen.
 15. Apparatus as in claIm 14 including means for scanningthe detection system image screens to simulate a scan of the object. 16.Apparatus as in claim 15 wherein the scanning means include means forsimulating a three-dimensional scan of the object.
 17. Apparatus as inclaim 15 wherein each of the detection systems includes an imageintensifier forming said image screen and wherein each detection systemincludes a video camera for electronically scanning said image screensto simulate a scan of the object.
 18. Apparatus as in claim 17 whereinsaid scanning means include means for simulating a three-dimensionalscan of the object.
 19. Method of examining an optically opaque objectcontaining penetrating radiation scattering centers, comprising thesteps of: a. irradiating the object with penetrating radiation to causeradiation scattering by scattering centers in the object; b. causingperiodic vibration of said scattering centers within the object; c.detecting radiation scattered by scattering centers in the object alongat least two noncongruent detection channels which are at an angle witheach other and which intersect at a common volume within the object; andd. correlating information generated by said detecting means in responseto detected scattered radiation with information representing theperiodic vibration of the scattering centers within the object to derivethereby information of internal physical and mechanical characteristicsof the object.
 20. Method as in claim 19 including the step of scanningthe object by the common volume at which the two detection channelsintersect.
 21. Method as in claim 20 wherein said scanning is carriedout by causing relative motion between the detection means and theobject.
 22. Method as in claim 19 wherein the detecting step includesimaging radiation along each of said detection channels to providethereby a pair of screen images, each screen image representingradiation scattered along one of said detection channels and each havinga plurality of points representing radiation scattered along portions ofthe detection channel, with selected pairs of points, one on eachscreen, defining radiation scattered from a common volume of the object.23. Method as in claim 22 including the step of successively examiningdifferent selected pairs of points of the screens to simulate a scan ofthe object.
 24. Method as in claim 23 wherein the correlating stepincludes correlating parameters of the selected pairs of points with thevibrating to enhance differences between different pairs of points. 25.Method of examining an optically opaque object which containspenetrating radiation scattering centers undergoing periodic vibrationalmotion, comprising the steps of: a. irradiating the object withpenetrating radiation to cause radiation scattering by said periodicallyvibrating scattering centers in the object; b. detecting radiationscattered by scattering centers in the object along at least twononcongruent detection channels which are at an angle with each otherand which intersect at a common volume within the object; and c.correlating information generated by said detecting means in response todetected scattered radiation with information representing the periodicvibration of the scattering centers within the object to derive therebyinformation of internal physical and mechanical characteristics of theobject.
 26. Method as in claim 25, including the step of causing a scanof the object by said common volume.
 27. Method as in claim 25 whereinthe detecting step includes imaging on image screens radiation scatteredalong each of said detection channels, with selected pairs of points,one on each detection image screen defining radiation scattered from acommon volume of the object.
 28. Method as in claim 21 including thestep of scanning the object by successively examining different selectedpairs of points on the image screen to simulate a scan of the object.